Preclinical data suggest that benzodiazepine (BZ) receptors are involved in the pathophysiology of anxiety and fear. However, assessment of BZ receptor function in humans has been limited, in part because of the tools available for direct assessment of BZ receptors in living human brain. We have recently obtained preliminary data in non-human primates supporting the feasibility of quantifying BZ receptors in vivo using single photon emission computerized tomography (SPECT) with the receptor antagonist [1231]iomazenil (Ro 16-0154). This radiotracer has excellent properties for in vivo imaging: high brain uptake, reversible binding to the receptor, high affinity, low non-specific binding, and favorable dosimetry. In the present application, we propose to develop and evaluate comparable quantitative receptor imaging methods for human subjects, with the outcome measure being the estimate of BZ receptor binding potential (B-max/Kd). Our initial attempts to quantify BZ receptor in human and non-human primates used three compartment four parameter modeling of the time course of regional brain activities and the plasma levels of parent compound. Because quantitation of SPECT neuroreceptor imaging is at an early and developing stage, we have emphasized studies in healthy subjects to better understand the inter- and intrasubject variabilities of the several important parameters, including the clearance of tracer from plasma, uptake and washout from brain, and estimation of binding potential. To assess the accuracy of this kinetic SPECT methodology, we propose to develop an equilibrium approach with [123I]iomazenil. In both human and non-human primates, we have established a prolonged state of apparent equilibrium in brain with the use of bolus plus constant infusion of [123I]iomazenil. Under equilibrium conditions, binding potential can be measured from a single SPECT scan and a single venous blood sample. We propose to compare binding potential estimates determined in the same subjects using both the "bolus only" (i.e., dynamic) and the "bolus plus infusion" (i.e., equilibrium) methods. The validity of SPECT quantification of BZ receptors will also be assessed relative to comparable PET imaging of [11C]iomazenil. We have obtained the necessary desmethyl precursor and have synthesized [11C]iomazenil with high specific activity. The use of the same chemical compound which differs only in radioisotopic substitutions may prove extremely valuable in comparing the relative abilities of PET and SPECT neuroreceptor imaging. The overall comparison of dynamic SPECT, equilibrium SPECT, and dynamic PET will be helpful in selecting the best method to investigate BZ receptors in patients with anxiety disorders.